The present invention deals with a method of refining platinum group metals by an electrochemical process.
The conventional method of refining platinum and platinum group metals from such compositions is to dissolve the metal in boiling nitric acid and hydrochloric acid (aqua regia) until it dissolves. A hydrazine compound (such as hydrazine hydrochloride, hydrazine acetate, hydrazine hydrate, hydrazine, hydrazine sulfate, etc.) is then used to precipitate the pure platinum compounds out of solution. This standard method is extremely hazardous, requires exceptional safety precautions and creates toxic waste.
Electrolytic processes for the refining of gold are known in the art. In my U.S. Pat. Nos. 4,612,093 and 4,895,626 I describe my discovery of a refining process (and improvements on that process) based on a novel "pregnant" electrolyte, comprising a halide solution impregnated with a nascent oxygen source. This process is practiced in an electrolytic cell, wherein the gold to be recovered is the anode. The cell cathode is an inert electroconductive substance isolated from the major portion of the electrolyte by a semipermeable membrane. This membrane is permeable to sodium ions but not to the electrolysis-dissolved gold contained in the electrolyte. Thus, the gold does not deposit on the cathode but remains in solution. Portions of the electrolyte with the gold solution are removed from the cell and the gold is selectively precipitated by chemical reduction.
In my U.S. Pat. No. 5,009,755 I describe a further improvement on the previous patent by use of an electrolyte of ammonium chloride. The ammonium chloride as an electrolyte has the benefits Of maintaining a pH neutral solution through a buffering effect and also dissolving a portion of the normally insoluble silver chloride that typically forms when jeweler's scrap gold is being refined.
In contrast in the present process ammonium ions react with platinum group metals to form an ammonium chloride platinum group salt from which the platinum group metal is ultimately recovered by the use of ammonium hydroxide.